Scientists harness firefly technology to make LEDs more efficient

Share This article

To skip a flat rock across the surface of a pond, it needs to be thrown at an angle shallow enough to bounce back into the air, otherwise, it is headed straight to the bottom. When light travels from a denser, higher refractive index medium to a less dense one, it similarly will be reflected if the the angle relative to vertical is greater than a critical angle. This phenomenon, known as total internal reflection, imposes significant restriction on the efficiency with which LEDs can transmit light into the air. By mimicking fireflies, which have solved the problem of optimally transmitting light from their mantles eons ago, researchers have shown it is possible to extract 55% more light from a GaN (Gallium Nitride) LEDs just by adding a single layer of material.

The problems facing LED manufacturers are more acute than those facing the firefly because the refractive index of semiconductors, like GaN, is much higher than that of the firefly’s light organ. This results in greater transmission mismatch to a lower index material and therefore more reflection. A semiconductor with a refractive index of 3.3, for example, would totally reflect any light incident at an angle greater than 17 degrees from vertical. That is a pretty narrow emission cone. The traditional method of smoothing these rough transitions is to add an intermediate layer or two having a refractive index somewhere in between. This method adds cost and complexity, and ultimately falls short because it ignores the most powerful tool we might use to control light — geometry.

The entrancing sparkle of hundreds of fireflies over a large field on a warm summer night gives little hint at the complex adaptations within their delicate mantle. The firefly produces its narrow-spectrum 560 nanometer light just like a chemical laser, but with even greater control over the oxidation reaction that powers it. The cuticle protecting the light-producing organ needs to be strong, yet sufficiently transparent to transmit light without significant loss.

There is also a layer of muscle over the mantle which has been selected for transparency and refractive properties which may be involved in the control the light pulse frequency or directionality. Examination of the mantle cover with an electron microscope has revealed the key: tiny serrations organized like a corrugated roof that repeats every 250 microns and gives it the appearance of a Fresnel lens.

The researchers make reference to “blind natural selection” as the author of this machinery, but in reality it is anything but blind. Fireflies may not understand all of the details, but the lady fireflies do understand one thing: weak lights are not sexy. Any time the males come out with a cuticle geometry that lets them throw off more of their metabolized light, the benefits do not go unnoticed.

Life has been generating and detecting light, arguably, since the very beginning. The lens of the eye, for example, is similarly a highly optimized and controlled device. The proteins that comprise our lens, known as crystallins, require a continuous feed of metabolic energy to maintain their transparency — any prolonged interruptions and cataracts invariably result. The lens tailors its composition and refractive index carefully across its extent to focus incoming light on the retina while minimizing errors introduced by things like spherical aberration. That the firefly dispenses its hard-won light with similar care that the eye takes to receive it should not surprise.

Understanding firefly scales as tiny prisms that change the way light impinges on an interface and creates new sharp-edged channels through which light can diffuse lets us make LEDs more efficient. Peering into nature with modern tools and understanding continues to offer great reward for addressing design issues shared in common. Features in nature may sometimes appear frivolous or even fickle, like the feathers of a peacock, but upon closer inspection usually offer something we can harness to improve the things we build.

Use of this site is governed by our Terms of Use and Privacy Policy. Copyright 1996-2015 Ziff Davis, LLC.PCMag Digital Group All Rights Reserved. ExtremeTech is a registered trademark of Ziff Davis, LLC. Reproduction in whole or in part in any form or medium without express written permission of Ziff Davis, LLC. is prohibited.